The present invention relates to gyroscopes utilizing a pair electromagnetic waves injected into a closed-ring resonant path, in opposite directions, in order to sense inertial rotation. The waves are caused to propagate around the ring path by means of some form of electromagnetic waveguide. More particularly, the invention relates to passive ring resonator gyros wherein the difference between resonant frequencies of two counter-propagating electromagnetic waves, introduced into the passive ring resonator path from a source of energy external to the ring path, is linearly proportional to the rotation rate.
The well known Sagnac Effect quantitatively describes the existence of a difference in the relative phase of two electromagnetic waves after traveling in opposite directions around a closed ring path. In gyro applications of the Sagnac Effect, the difference in phase is directly proportional to the inertial rotation rate about an axis normal to the plane of the ring path. Because the ring path is closed, the waves may propagate in multiple round trips around the ring. The waveguide ring structure is resonant at certain optical frequencies. Because of the Sagnac phase difference, the resonant frequencies are different for the waves propagating in the opposite directions in the presence of inertial rotation due to differences in the effective closed-loop path length traveled by the two counter-propagating waves. This frequency difference is directly proportional to the inertial rotation rate which may be measured by measuring the difference in the resonant frequencies of the counter-propagating waves. The frequency difference and sensitivity of the gyro to rotation is further a function of the area circumscribed by the closed ring waveguide path, the perimeter of the waveguide path, the effective refractive index of the waveguide an material within the waveguide, and the free-space wavelength of the waves.
In the prior art of passive ring resonator gyros, operation has been generally restricted to utilizing a single-polarization wave in each direction and nearly always the same polarization in both directions. In general, electromagnetic waveguides are capable of supporting propagating waves of two possible polarizations, the most common of which are linearly polarized horizontal and vertical waves, but in some systems may be a pair of elliptically polarized waves or even right and left circularly polarized waves.
Similarly, in a ring resonator composed of such electromagnetic waveguide material, two polarization states exist in each direction around the ring. These states, each of which have resonances associated with them, are commonly referred to as resonant polarization eigenstates. Polarization eigenstates are essentially states of polarization that at any point around the ring do not change their state of polarization after one or more round trips around the ring. Eigenstates of polarization are described in detail in a publication entitled, "Eigenstates of Polarization in Lasers" by H. de Lang, Philips Research Repts, Vol. 19, pp. 429-440, 1964; and a publication entitled, "Marix Method for the Calculation of the Polarization Eigenstates of Anisotropic Optical Resonators", by V. Ya. Molchanov and G. V. Skrotskii, Soviet Journal of Quantum Electronics, Vol. 1, No. 4, pp. 315-330, January-February, 1972. Further, Eigenstates of polarization in a passive ring resonator gyros are discussed in a publication entitled, "Eigenstates of Polarization in a Fiber Ring Resonator and its Effect in an Optical Passive Ring-resonator Gyro", by K. Iwatsuki, K. Hotate, and M. Higashiguchi, Appl. Opt. 25, 15, 2606-12 (1986). These references are incorporated herein by reference.
Eigenstates of polarization can be simple, such as nearly horizontal and vertical linear polarized waves, elliptical polarized waves, or can exhibit complex behavior in which the polarization varies from point to point along the resonator.
Ring resonators of the prior art are often those having very nearly horizontal and vertical linear polarization eigenstates. In ring resonator gyros of the prior art, the possible excitation of both polarizations and the possible nearly identical resonant frequencies for the two different polarizations can prevent gyro operation or compromise its performance. To alleviate this problem, one of the two polarized waves in each direction is usually suppressed by some means or by providing special means so that the external source does not excite the other unwanted polarization. Prior art schemes for obviating the problem of unwanted polarized waves rely on, among others, careful polarization control of the light waves, tight component tolerances, and/or the introduction of high propagation losses for one of the two polarization eigenstates. These schemes are generally costly to implement, complex, and/or may introduce other error sources.